The present invention relates generally to telecommunication technologies. More particularly, an exemplary embodiment of the present invention relates to a method of modulating and demodulating a communication signal using differential quadrature phase shift keying (DQPSK).
In general, modulation is the process of impinging information (e.g., voice, data, or image) on a carrier signal and demodulation is the recovery of that information from the carrier at the distant end near a destination user. The equipment that performs the necessary conversion of a communication signal is typically called a modem, an acronym for modulator-demodulator. A modem modulates and demodulates a carrier signal with digital data signals. One type of modulation is phase modulation, which corresponds in digital terminology to phase shift keying (PSK).
One form of PSK is binary PSK. In binary PSK, there are two phases separated from each other by 180° phase difference. The demodulator is based on a synchronous detector using a reference signal of known phase. This known signal operates at the same frequency of the incoming signal carrier and is arranged to be in phase with one of the binary signals.
Digital personal communication service (PCS) is a popular name for a cellular standard for Time Division Multiple Access (TDMA). The actual Digital PCS standard is known as TIA/EIA-136 and was developed by the Telecommunications Industry Association (TIA). The modulation format of the TIA/EIA-136 digital control channel and digital traffic channel is Pi/4 shifted, differentially encoded quadrature phase shift keying (Pi/4 DQPSK).
In general, a differential quadrature phase shift keying (DQPSK) modulator operates as follows. First, the modulator receives binary data and converts it to a parallel stream. Then, the parallel streams are differentially encoded into two bit symbols using a symbol rate of, for example, 24.3 k symbols/second. The resulting two encoded data sequences are passed through square-root raised cosine filters for pulse shaping, and then used to quadrature modulate a carrier. The differential encoding of the data is carried out as differential phase encoding according to the rules shown in Table 1 below.
TABLE 1XYTheta00Pi/4013 Pi/410−Pi/411−3 Pi/4This encoding allows for differential detection of the data at the receiver and limits the phase changes to minimize the linearity requirements of power amplifiers in the transmit chain. As such, the differential phase encoder maps two consecutive bits, X and Y, to the Real and Imaginary component of the output signal. The mapping of X and Y can be shown as: real(S)=2*X−1 and imag(S)=2*Y−1.
Accordingly, conventional Pi/4 DQPSK modulation can be described using the formula:S(t)=e(J*Theta)*e(j*t*Pi/4)where Theta is defined according to the encoding rules in Table 1 above. Every time step, new data (X, Y) is used to calculate the output Symbol.
As such, the conventional modulation scheme requires a complex multiplication, involving real and imaginary parts. In practice, such a complex multiplication is computationally challenging, requiring components which are expensive and complex to carry out the mathematics.
Thus, there is a need for modulating and demodulating a communication signal using an improved differential quadrature phase shift keying (DQPSK) method. Further, there is a need for more simple technique for translating Pi/4 DQPSK symbols into QPSK symbols. Yet further, there is a need for an optimized and resource saving DQPSK method.